Medications used to treat psychotic disorders are called antipsychotics. Typical antipsychotics (sometimes referred to as conventional antipsychotics) are class of first generation antipsychotic drugs and used to treat psychosis including schizophrenia. The typical antipsychotics include chlorpromazine (THORAZINE®), fluphenazine (PROLIXIN®), haloperidol (HALDOL®), thiothixene (NAVANE®), trifluoroperazine (STELAZINE®), perphenazine (TRILAFON®), and thioridazine (MELLARIL®). The second generation antipsychotics introduced in the 1990's are called atypical antipsychotics. Compared to the first generation antipsychotics, the atypical antipsychotics appear to be equally effective in reducing the positive symptoms like hallucinations and delusions but may be better than the typical antipsychotics at relieving the negative symptoms of schizophrenia such as apathy, withdrawal, emotional depression and the like. The atypical antipsychotics currently in clinical use include Aripiprazole (ABILIFY®), clozapine (CLOZARIL®), risperidone (RISPERDAL®), olanzapine (ZYPREXA®), quetiapine (SEROQUEL®), and ziprasidone (GEODON®).
Atypical antipsychotics have diminished propensity to cause extrapyramidal symptoms (EPS) and tardive dyskinesia (TD) than typical antipsychotics. Additional benefits associated with the atypical antipsychotics include better treatment of negative symptoms, better compliance, possible benefits for cognitive impairments, and lower rates of relapse. Within the class of atypical antipsychotics, however, differences exist both in efficacy and side effects. Clozapine does not cause EPS, and is clearly more effective than all other antipsychotics used in humans to date. It is however a life-altering drug, because of its side effects and need for continual medical monitoring, in some countries, for agranulocytosis. This has markedly limited its use. The other atypical antipsychotics with the greatest amount of efficacy data are risperidone and olanzapine. These drugs are the most commonly used first-line antipsychotics today. This is warranted because they are more clinically effective than conventional drugs and much easier to use than clozapine. However, both risperidone and olanzapine are limited by side effects. Risperidone causes prolactin elevations, weight gain and dose-dependant EPS. Olanzapine use is associated with much more weight gain in addition to lipid and glucose abnormalities. Qetiapine and Ziprasidone may be safer alternatives to risperidone and olanzapine but these drugs do not appear to be as clinically effective as the other atypical antipsychotics. Aripiprazole is one of a new generation of atypical antipsychotic drugs approved by the FDA for the treatment of schizophrenia in November 2002 (Satyanarayana, C. et al. WO 2006/030446; Tsujimori, H. et al. WO 2004/063162; Salama, P. et al. WO 2004/099152; Wikstorm, H. et al. WO 2003/064393). It was approved for the treatment of acute mania and mixed episode associated with bipolar disorder in March 2005. Aripiprazole does not differ greatly from other atypical antipsychotics with respect to treatment response, efficacy and tolerability.
Atypical antipsychotics are increasingly being used in children and adolescents for a variety of psychiatric conditions. Conditions for which atypical antipsychotics are prescribed include bipolar disorder, psychotic depression, schizophrenia, pervasive developmental disorders, attention-deficit/hyperactivity disorder (ADHD), oppositional defiant disorder (ODD), and conduct disorder. They are also used symptomatically to treat rage, insomnia, and anorexia. Younger patients appear to be at a higher risk of adverse effects associated with the treatment of atypical antipsychotics especially weight gain and drug induced diabetes mellitus.
In general, atypical antipsychotics share many of the side effects of typical antipsychotics, including sedation, akathisia, weight gain, extrapyramidal symptoms (EPS), neuromalignant syndrome, and tardive dyskinesia; longer experience with them have shown that new risks need to be considered, such as metabolic syndromes and QTc prolongation. QTc prolongation is known to have potential liability to produce fatal cardiac arrhythmias of Torsades de Pointes (TdP). Drug induced adverse metabolic effects such as weight gain, lipid abnormalities, and diabetes mellitus have been identified as a major risk factor for various medical disorders that might be responsible for some of the increased morbidity and mortality rates in psychotic patients treated with atypical antipsychotics.
Off-target pharmacology and drug to drug interactions are mainly responsible for most of the adverse side effects associated with the atypical antipsychotics. All the atypical antipsychotic drugs currently being used for the treatment of schizophrenia and related psychotic disorders have poor therapeutic target selectivity. For example, one of the most widely prescribed atypical antipsychotic drugs, Olanzapine and the most effective atypical antipsychotic drug, clozapine are reported to have significant activities against more than 12 receptors such as dopamine (D1, D2, D3 and D4), serotonin (5-HT2A, 5-HT2C, 5-HT6, and 5-HT7), adrenergic (alpha 1 and alpha 2), histamine (H1), muscarinic (M1), Dopamine transporter (DAT) and norepinephrine transporter (NET) receptors (Miyamoto et al., Molecular Psychiatry, 2005, 10, 79). Similarly, the other FDA approved atypical antipsychotics such as risperidone and aripiprazole are also reported to have significant activities against more than nine of the receptors mentioned above. The current reasearch suggests that compounds exhibiting activity against dopamine (D2) and serotonin (5-HT1A and 5-HT2A) receptors may have the intended antipsychotic effect (Snyder, S. H., Nature 2008, 452, 38-39; Di Pietro, N. C., Seamans, J. K., Pharmacopsychitry 2007, 40(S1), S27-S33; Stark, A. D. et al., Psychopharmacology 2007, 190, 373-382) while compounds exhibiting activity against other receptors like serotonin, 5HT2C, histamine (H1), and adrenergic (alpha 1) may cause adverse side effects such as cardiac arrhythmias.
Although, the atypical antipsychotics (aripiprazole, clozapine, risperidone, olanzapine, quetiapine, and ziprasidone) currently in clinical use represent significant advances in treatment of people with schizophrenia, there is a need for new psychotropic drugs with improved safety profiles.
Therefore, development of a novel antipsychotic that has improved therapeutic target selectivity than the currently available therapies would provide effective and safer medicines for the treatment of schizophrenia and related psychotic disorders.